This invention relates to motor controls, and in particular, to a method of controlling the starting, stopping and speed of an AC induction motor with a soft starter.
There are two basic approaches for controlling the starting, stopping and speed of an AC induction motor. In a first approach, an adjustable frequency controller is interconnected to the AC induction motor. The adjustable frequency controller is comprised of an inverter which uses solid state switches to convert DC power to stepped waveform AC power. A waveform generator produces switching signals for the inverter under control of a microprocessor. While adjustable frequency controllers efficiently control the motor speed and the energy used by an AC induction motor, use of such types of controllers may be cost prohibitive. Further, since many applications of AC induction motors do not require sophisticated frequency and voltage control, an alternative to adjustable frequency controllers has been developed.
An alternate approach to the adjustable frequency controller is the soft starter. Soft starters operate using the principal of phase control whereby the three phase main supply to the AC induction motor is controlled by means of anti-parallel thyristor switches in each supply line. In phase control, the thyristor switches in each supply line are fired to control the fraction of the half cycle over which current is conducted to the motor (known as the conduction period). The non-conducting period of each half cycle (known as the hold-off angle or the notch width) is visible as a notch in the voltage waveform at each motor terminal. During this period, no current flows to the motor terminals. To end the non-conducting period, the thyristor switches in the supply line to the motor terminals are fired to restart their conduction. The conduction through the thyristor switches continues until the current, once again, becomes zero at some point in the next half cycle and the thyristor switches reopen. According to the principles of phase control, by varying the duration of the non-conducting period, the voltage and current supplied to the AC induction motor may be controlled.
Heretofore, the use of a soft starter to start an AC induction motor has resulted in excessive acceleration or deceleration of the AC induction motor during certain applications. With loads that are driven by a belt, the rapid increase in torque with speed can cause the belt to slip on the motor pulley thereby producing severe wear of he belt. Consequently, it is highly desirable to provide a method of smoothly starting nd stopping an AC induction motor with a soft starter without the instability of prior methods.
Further, soft starters often incorporate a bypass contactor in parallel with a corresponding thyristor switch in the supply line. Once the AC induction motor reaches a predetermined operating speed, the bypass contactors are closed such that the AC induction motor is connected directly to the AC source through the bypass contactors. It has been found that, for a variety of reasons, one or more of the bypass contactors may fail during operation of the AC induction motor. In order to maintain the voltage and current to the AC induction motor during failure of a bypass contactor, the thyristor switches on each supply line are periodically fired every few seconds. However, due to the potential delay between the opening of the bypass contactor and the firing of the thyristor switch, arcing may occur between the contacts of the opening bypass contactor. As is known, the magnitude of the arcing increases as the time period increases between the opening of the bypass contactor and the firing of the thyristor switch. As a result, the bypass contactors used in soft starters are often rated for four to six times the full load amperes (FLA) of the AC induction motor. Use of such large bypass contactors not only increase the size of the soft starter, but the cost of manufacturing the same. Consequently, it is highly desirable to provide an improved method for firing the thyristor switch in response to the opening of a corresponding bypass contactor so as to allow for the use of a bypass contator in a soft starter which is rated at approximately at the FLA of the AC induction motor to be started.
Therefore, it is a primary object and feature of the present invention to provide an improved method for controlling the starting, stopping and speed of an AC induction motor.
It is a further object and feature of the present invention to provide a method for controlling an AC induction motor which provides smooth and gradual acceleration and deceleration of the AC induction motor.
It is still a further object and feature of the present invention to provide a method for controlling the starting, stopping and speed of an AC induction motor which is less expensive to manufacture.
In accordance with the present invention, a method for controlling a three phase, AC induction motor is provided. Each phase of the induction motor is interconnected to an AC source by a thyristor switch and a bypass contactor for providing voltage and current to the AC induction motor. The method includes the steps of sequentially firing pairs of thyristor switches to bring the AC induction motor to full operating speed. Each thyristor switch opens in response to a zero current supplied from the AC input source. The operating speed of the AC induction motor is monitored and the bypass contactors are closed in response to the AC induction motor rotating at a predetermined operating speed and/or the starting current having fallen below the FLA for the AC induction motor. After the closing of the bypass contactors, the thyristor switches remain open unless a voltage drop is detected across one of them. Upon detection of a voltage drop across the thyristor switch, the thyristor switch is immediately fired.
The step of firing the pairs of thyristor switches may include the step of providing constant current to the AC induction motor or the step of increasing the current to the AC induction motor by each subsequent firing of the pairs of thyristor switches.
It is contemplated to monitor the temperature on each phase of the AC induction motor and to stop the AC induction motor in response to the temperature on one of the phases of the AC induction motor exceeding a predetermined temperature. The AC induction motor may also be monitored for thermal overload condition wherein the AC induction motor is stopped in response thereto. It is still further contemplated to monitor the voltage supplied to the AC induction motor and to stop the motor in response to the voltage on any phase of the AC induction motor being below a predetermined value.
The induction motor may be stopped in response to a user stop command. The step of stopping the induction motor includes the steps of opening the bypass contactors and reducing the current to the induction motor for a user selected time. Thereafter, the thyristor switches open such that the induction motor may stop under load.
In accordance with a further aspect of the present invention, a method for controlling the voltage and current supplied to an AC motor is provided. The AC motor is connected to an AC input source through a thyristor switch and a bypass contactor connected in parallel. The method comprises the steps of determining an initial occurrence of zero supply voltage at a motor terminal and firing the thyristor switch at a predetermined firing angle after the initial occurrence of the zero supply voltage at the motor terminal The predetermined firing angle is provided as an initial firing angle. A subsequent occurrence of a zero supply voltage at the motor terminal and an initial occurrence of a zero supply current to the motor terminal from the AC source are determined. The delay between the subsequent occurrence of the zero supply voltage at the motor terminal and the initial occurrence of the zero supply current to the motor terminal is provided as an initial phase lag. The thyristor switch opens in response to the initial occurrence of the zero supply current, and thereafter, the thyristor switch is fired, once again, at the initial firing angle after the subsequent occurrence of the zero supply voltage at the motor terminal. A next, subsequent occurrence of a zero supply voltage at the motor terminal and a subsequent occurrence of a zero supply current to the motor terminal is determined. The delay between the next, subsequent occurrence of the zero supply voltage at the motor terminal and the subsequent occurrence of the zero supply current to the motor terminal is provided as a new phase lag. The thyristor switch reopens in response to the subsequent zero supply current therethrough. A new firing angle is calculated in response to the initial firing angle and the difference between the initial phase lag and the new phase lag. The thyristor switch is then refired at the new firing angle.
The method may also include the additional steps of monitoring the operating speed of the motor and closing the bypass contactor in response to the AC motor rotating at a predetermined operating speed and the starting current being below the FLA of the AC motor. After the closing of the bypass contactor, the thyristor switch may be fired in response to a voltage drop thereacross.
The AC motor may be stopped in response to a user command. The step of stopping the motor includes the steps of opening the bypass contactor and reducing the current supplied to the AC motor for a user selected time. Thereafter, AC induction motor is switched off by discontinuing any further firing of the thyristor switches.
The method for controlling the voltage and current supplied to the AC motor may also include the steps of providing the new firing angle as the initial firing angle, providing the new phase lag as the initial phase lag, and returning to the step of determining a next, subsequent occurrence of a zero supply voltage at the motor terminal.
In accordance with a still further aspect of the present invention, a method is provided for controlling an AC motor interconnected to a AC input source by a thyristor switch and a bypass contactor connected in parallel for providing voltage and current to the AC motor. When operating properly, the bypass contactor is closed when the AC motor is at full operating speed. The method includes the steps of operating the AC motor at full operating speed and firing the thyristor switch in response to a voltage drop thereacross.
The step of operating the AC motor at full operating speed may include the steps of applying voltage and current to the AC motor and monitoring the operating speed of the AC motor. The bypass contactor is closed in response to the AC motor rotating at a predetermined operating speed.
The step of applying voltage and current to the AC motor includes the steps of determining an initial occurrence of zero supply voltage at a motor terminal and firing the thyristor switch at a predetermined firing angle after the initial occurrence of the zero supply voltage at the motor terminal The predetermined firing angle is provided as an initial firing angle. A subsequent occurrence of a zero supply voltage at the motor terminal and an initial occurrence of a zero supply current to the motor terminal from the AC source are determined. The delay between the subsequent occurrence of the zero supply voltage at the motor terminal and the initial occurrence of the zero supply current to the motor terminal is provided as an initial phase lag. The thyristor switch opens in response to the initial occurrence of the zero supply current, and thereafter, the thyristor switch is fired, once again, at the initial firing angle after the subsequent occurrence of the zero supply voltage at the motor terminal. A next, subsequent occurrence of a zero supply voltage at the motor terminal and a subsequent occurrence of a zero supply current to the motor terminal is determined. The delay between the next, subsequent occurrence of the zero supply voltage at the motor terminal and the subsequent occurrence of the zero supply current to the motor terminal is provided as a new phase lag. The thyristor switch reopens in response to the subsequent zero supply current therethrough. A new firing angle is calculated in response to the initial firing angle and the difference between the initial phase lag and the new phase lag. The thyristor switch is then refired at the new firing angle.
The method may also include the additional steps of providing a new firing angle as the initial firing angle, providing the new phase lag as the initial phase lag, and returning to the step of determining the subsequent occurrence of zero supply voltage to the AC motor.
It is further contemplated that the method include the step of stopping the AC motor in response to a user command. The step of stopping the AC motor includes the steps of opening the bypass contactor; transferring current to the thyristor switch; and reducing the current supplied to the AC motor for a user selected time. Thereafter, the thyristor switch is left open to disconnect the AC motor. The method may also include the steps of monitoring the AC motor for a thermal overload condition thereon or a reduction in the voltage supplied to the AC motor by the AC source. In the event of such conditions, the AC motor is stopped.